DD's got a few and they've changed a bit over the months. When she really little it would be the vacant stare that was a bit of a giveaway but that was quite subtle. Then she went through a phase of sucking her thumb when she was tired which was fantastic in terms of being able to spot it.

Now, at 10 months, she's got a combo of the vacant stare again OR getting a bit manic with her toys and wanting to play with everything at the same time. That one's a bit deceptive because if you didn't know her, you'd say she was full of beans and not at all ready for bed.